The objective is to test a new theory relating mechanics and energetics of locomotion. The theory is based on the invariant material properties of the structures involved in support and movement (muscle, bone and tendon), and the observation that the same mechanical mechanisms are utilized to sustain a constant speed along the ground. It assumes that muscles are used in a manner that minimizes the cost of generating force on the ground, and that most of the mechanical energy changes of the body are conserved in pendulums and springs. A consequence is that muscles do little work once a runner reaches a constant speed. The theory predicts how the various locomotory parameters (e.g., energy cost of running, efficiency of running up inclines, cross-sectional area of active muscles, stride frequency, rate of force development, and properties of the springs) change as functions of gait, numbers of limbs, and body size. It explains the 20-fold difference in the mass specific energy cost of running a mile between a mouse and a horse, and accurately predicts the rate of energy consumption of running quadrupeds over their entire speed range; while theories assuming muscles Of running animals are designed to work efficiently do not. Our goal is to test both the assumptions and the predictions of the theory and to refine it as necessary. Specifically, the aims of the project are: i) to extend the model quantitatively to bipedal running in humans and birds by measuring mechanical advantage and fiber length of the major muscles used in support; ii) to extend the theory from running to walking and to test the predictions in both bipeds and quadrupeds; iii) to test the predictions of the model for muscle activity, and mechanical efficiency as pendulums and springs are quantitatively removed from the system (using incline walking and running); and iv) to test the assumption that muscles perform little work during running. Mechanical advantage of the support muscles will be measured using a force platform/high speed video analysis technique in conjunction with detailed anatomical measurements. Rate of energy consumption of the muscles will be calculated from measurements of oxygen consumption of animals walking/running on treadmills. Rate of force development will be calculated from time of foot contact. The length changes of major support muscles during running will be measured directly using a sonomicrometer. The theory runs counter to conventional wisdom about how muscles are used in locomotion and in every day activities. It has important consequences for physical, occupational and rehabilitative therapy; for the design of prostheses; for training; and for selecting appropriate animal models for experiments involving the musculoskeletal system. Increasing our understanding of the unifying principles in the design of the system is central to avoiding failure and improving performance, whether it be for rehabilitating the injured or training elite athletes.